(1) Field of the Invention
This invention relates to the use of smectite clay composites containing alkaline earth metal hydroxides and carbonates for the removal of SO.sub.x, (sulfur dioxide and sulfur trioxide), from flue gases, particularly flue gas from coal burning power plants, and to the method of preparing them.
(2) Prior Art
The first example of flue gas scrubbing for sulfur dioxide control occurred in London, England, in 1933. However, the application of this technology to coal-fired utility boilers in the United States did not begin until the 1970's. The first large-scale application of flue gas scrubbing using lime or limestone was installed in 1964, in the Soviet Union. This facility was followed by an installation at a large sulfuric acid plant in Japan in 1966. In 1970, the Clean Air Act Amendments were adopted. This legislation provided for enforcement, by the United States Environmental Protection Agency (EPA), of SO.sub.x emissions limits for power plants constructed or modified after Aug. 17, 1971. This Act spurred extensive flue gas desulfurization (FGD) research. As of January 1984, calcium based, wet, throwaway systems (including lime, limestone, and alkaline-ash systems) accounted for 84 percent of existing and planned FGD capacity. The Clean Air Act was amended in 1977 to require further control of SO.sub.x emissions. Increasing federal regulations and the high cost to construct and operate existing wet FGD units have encouraged continued research on new or modified flue gas cleanup processes.
Controlling the emissions of SO.sub.x from power plants is a world-wide problem due to its relationship to "acid rain". Therefore, research into its control is a global effort. Example of a recent patent using calcium based systems to reduce SO.sub.x emissions is Thompson and Nuzio, U.S. Pat. No. 4,731,233. In most cases a commercial source of limestone or lime is used due to cost effectiveness and available quantities.
There are a number of ways to control SO.sub.x emissions in existing power plants or features that can be included in construction of new power plants. These approaches can be classified according to the position in the combustion system at which pollutant control technology is applied. Precombustion control involves removal of sulfur, nitrogen and ash compounds from the fuel before it is burned. In most cases this involves application of coal-cleaning technology. Combustion control includes staged combustion, boiler limestone injection, and fluidized-bed combustion with limestone addition. Post-combustion control involves removal of pollutants after they have been formed but before they are released into the atmosphere. This would include in-duct dry sorbent injection, induct spray drying and combined electrostatic precipitator (ESP)/fabric filter sorbent injection (Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React. Solids, 6 243 (1988)).
U.S. Pat. No. 4,981,825 to Pinnavaia and Moini, describes the use of smectite clays mixed with metal oxide sol particles to prevent sintering of the clays when heated to elevated temperatures. This method is more complicated than necessary with calcium hydroxide, calcium oxide or other metal oxides which are reactive with SO.sub.x.
Flue gas treatment systems can be classified as either wet or dry based on the moisture content of the treated flue gas and the spent sorbent. Wet systems completely saturate the flue gas with water vapor. The flue gas is contacted with a liquid or slurry stream. Dry systems contact the flue gas with a dry or wet sorbent but never include enough water for complete saturation of the flue gas. Dry systems result in a dry product or spent sorbent material, while wet systems results in either a slurry or a sludge.
Although calcium based systems are the major source of SO.sub.x control, they are not without problems. Agglomeration of particles can be a serious problem that results in a less than optimal conversion to CaSO.sub.x, (CaSO.sub.3 and CaSO.sub.4). The activity of the calcium species decreases as its particle size increases. Also CaSO.sub.x occupies more volume than CaO, the common active species. Therefore, an increase in volume occurs as the reaction proceeds, which causes a loss in the original porous character of the CaO. This results in a blockage of SO.sub.x and O.sub.2 to the active CaO centers (Gullett, B. K. and Blom, J. A., React Solids, 3 337 (1987); Gullett, B. K., Blom, J. A. and Cunningham, R. T., React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30 450 (1984); Thibault, J. D., Steward, F. R. and Ruthven, D. M., Can. J. Chem. Eng., 60 796 (1982)).
Prior Art in this field has used limestone, lime or hydrated lime as a precursor for the active CaO species or has used Ca(OH).sub.2 as the active species. Generally, the active species has been used as a bulk phase and not as a dispersed species (Chang, J. C. S. and Kaplan, N., Envir. Prog., 3 267 (1984); Gullett, B. K., Blom, J. A. and Cunningham, R. T., React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30 450 (1984); Fuller, E. L. and Yoos, T. R., Langmuir, 3 753 (1987)). Recent work has concentrated on the addition of fly ash to Ca(OH).sub.2 to enhance its activity in SO.sub.x control (Jozewicz, W. and Rochelle, G. T., Envir. Prog, 5 219 (1986); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., JAPCA, 38 796 (1988); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React. Solids, 6 243 (1988); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., EPA/600;D-87/095, (NTIS PB87-175857/AS); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., EPA/600/D-87/135, (NTIS PB87-182663). The fly ash is a siliceous material and formation of various calcium silicates can occur. Several diatomaceous earths, montmorillonite clays and kaolins have also been identified as containing reactive silica (Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React. Solids, 6 243 (1988)).
Conventional systems use limestone, lime, or hydrated lime as a precursor for the reactive CaO species or have used Ca(OH).sub.2 as the reactive species. Agglomeration of particles is a serious problem that results in less than optimal conversion to CaSO.sub.x for all of these systems. The activity of the calcium species decreases as its particle size increases. This is caused by the larger volume that CaSO.sub.x occupies compared to CaO, the common active species. An increase in volume occurs as the reaction proceeds, which causes a loss in the original porous character of the CaO. This results in a blockage of SO.sub.x and O.sub.2 resulting in inefficient removal of SO.sub.x from flue gas streams (Gullett, B. K. and Blom, J. A., React. Solids, 3 337 (1987); Gullett, B. K., Blom, J. A. and Cunningham, R. T., React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30 450 (1984); Thibault, J. D., Steward, F. R. and Ruthven, D. M., Can. J. Chem. Eng., 60 796 (1982)).